1. Field of the Invention
The present invention relates to an input/output interface system called a SCSI (Small Computer System Interface) for connecting a host computer with input/output devices such as hard disk drives and CD-ROM drives.
2. Description of Related Art
FIG. 7 shows a conventional SCSI system. In FIG. 7, the reference numeral 101 designates a SCSI system. It comprises as its main components a SCSI controller 102, a SCSI cable 103 for interconnecting the SCSI controller 102 with SCSI devices 150 (150a-150n), such as hard disk drives and CD-ROM drives, and a terminator 104 for terminating the SCSI. The reference numeral 105 designates a SCSI cable 103 signal controller for controlling SCSI signals in the SCSI controller 102, 106 designates a terminator for terminating the SCSI in the SCSI controller 102, and 107 designates a bus in the SCSI controller 102. Reference numerals 150a-150n each designate a SCSI device such as a hard disk drive or CD-ROM drive, the reference numeral 151 designates a host computer, and 152 designates a bus interconnecting the host computer 151 with SCSI controller 102.
FIG. 8 is an enlarged diagram showing the SCSI cable 103 in the conventional SCSI system 101. In FIG. 8, reference numerals 111a-111n each designate a SCSI connector to which the SCSI device is connected, and the reference numeral 112 designates a cable for interconnecting the SCSI connectors 111a-111n. The cable 112 comprises a line (called "REQ# signal line" from now on) for conveying a REQ# signal and a line (called "ACK# signal line" below) for conveying an ACK# signal, where the symbol "#" denotes "active low", and hence the REQ# signal refers to an active low REQ signal, and the ACK# signal refers to an active low ACK signal. The SCSI connectors 111a-111n each include two unused pins. Incidentally, in FIG. 8, a data bus and the other control lines are not shown.
Next, the operation of the conventional SCSI system 101 will be described with reference to timing charts as shown in FIGS. 4A and 4B. Receiving a command to carry out data exchange with a target SCSI device 150 from the higher rank host computer 151 through the bus 152, the SCSI controller 102 first establishes a logical connection with the target SCSI device 150.
Then, the SCSI controller 102 carries out the data exchange. First, a case will be described in which data is read from the SCSI device 150. In the read operation, the SCSI device 150 transfers data to the SCSI controller 102 by placing it on the data bus of the SCSI in synchronism with the control signal REQ# as shown in FIG. 4A. The SCSI controller 102 samples the data at the rising edge of the REQ# signal. Subsequently, it sends back the control signal ACK# to the SCSI device 150 in synchronism with the REQ# signal to establish synchronization with the SCSI device 150. By repeating the foregoing operations, the SCSI device 150 transfers the data to the SCSI controller 102.
Likewise, when the SCSI controller 102 writes data to the target SCSI device 150, the SCSI device 150 first sends the REQ# signal to the SCSI controller 102. Upon recognizing the REQ# signal, the SCSI controller 102 asserts the ACK# signal in synchronism with this, and places the data on the data bus of the SCSI. The SCSI device 150 samples the data at the rising edge of the ACK# signal. By repeating the operations, the SCSI controller 102 transfers the data to the SCSI device 150.
Thus, the SCSI system transfers data in synchronism with the rising edge of the REQ# signal or ACK# signal. This means that these two signals must have a frequency twice that of the other SCSI signals. In addition, since the REQ# signal and ACK# signal are each interconnected in the same manner as the other signals in the cable 112 as shown in FIG. 8, its load capacitance will increase with the number of the SCSI devices connected to the SCSI connectors 111, thereby increasing the noise level caused by reflection or the like. This presents a problem in that normal operation cannot be ensured owing to increasing distortion in the REQ# signal or ACK# signal that will occur when the maximum of seven SCSI devices are connected in a modern fast SCSI system such as a Ultra SCSI system which transfers data at a high speed.
In view of this, Japanese patent application laid-open Hei. No. 9-6719 discloses an interface system with a repeater for solving the problem. This system, however, requires the repeater for interconnecting the maximum of seven SCSI devices, complicating the connection configuration. In addition, once the REQ# signal or ACK# signal is asserted LOW in the repeater, it is latched and cannot be returned to HIGH. This presents another problem of being unable to achieve the function as the SCSI system.